A lithium secondary cell having a cell element comprising a positive electrode, a negative electrode and an electrolytic solution, accommodated in a casing, is known.
The most common lithium secondary cell employs, as a casing, a metal can made of a metal such as SUS and having rigidity. In such a lithium secondary cell, a cell element is accommodated in a casing so that its electrodes are sufficiently close to each other.
In recent years, instead of such a lithium secondary cell employing a metal can, a lithium secondary cell employing a sheathing member having a variable shape, which is made of laminated film comprising a gas barrier layer and resin layers provided on both sides of said gas barrier layer, as a casing, has been practically used. In such a lithium secondary cell, it becomes possible to reduce the weight of the sheathing member and to reduce the thickness of the lithium secondary cell, and it becomes possible to further reduce the size and weight of the lithium secondary cell, and further to improve the volume energy density and the weight energy density.
However, with such a lithium secondary cell employing a casing having a variable shape, the strength of the casing is weak as compared with a lithium secondary cell sealed in a stout metal can casing, whereby the closeness between the electrodes is poor, and there are new problems such as deterioration of the cycle characteristics and deterioration of impregnation with the electrolyte.
Namely, in a lithium secondary cell employing a casing having a variable shape, the closeness between the electrodes of the cell element is secured usually by the atmospheric pressure by reducing the pressure inside the casing. However, the atmospheric pressure does not provide a sufficient force to press the cell element from outside, and consequently, the closeness between the electrodes deteriorates, whereby the cycle characteristics tend to deteriorate. Especially, if the degree of the reduced pressure inside the variable shape casing lowers due to e.g. deterioration of a bonded portion of the variable shape casing by high temperatures, interfusion of moisture due to this deterioration, generation of a gas by the decomposition of the electrolytic solution, or in the case a low boiling point solvent is used, evaporation of the solvent, the closeness between the electrodes further lowers and the cycle characteristics significantly deteriorate.
Whereas, in a lithium secondary cell employing a metal can casing, the metal can casing having high rigidity strongly presses the cell element accommodated therein, and the closeness between the electrodes is sufficiently high.
Further, if the variable shape casing is employed, since the closeness between the electrodes is inferior as compared with in the case of using a metal can casing, the impregnation property of the electrolyte significantly influences the cell performance. Namely, in a cell employing a conventional metal can casing, as mentioned above, the metal can casing plays a role of pressing the cell element from outside, whereby the closeness between the electrodes is essentially high, and accordingly, the influence of deterioration of the impregnation property of the electrolyte to the cell performance is insignificant. However, in a cell employing a casing having a variable shape, closeness between the electrodes is relatively inferior, since such a casing has low rigidity as compared with the metal can casing, and consequently the impregnation property of the electrolyte significantly influences the cell performance.
Deterioration of the impregnation property of the electrolyte is remarkable particularly in a case where a non-fluid electrolyte having its own prescribed independence is used. A lithium secondary cell having a non-fluid electrolyte is produced usually via a step of impregnating e.g. electrodes with a solution containing an electrolytic solution, a monomer and a polymerization initiator, or with a solution containing an electrolytic solution and a polymer. However, the above-mentioned solution can hardly be impregnated into the electrodes, since it has a high viscosity as compared with an electrolytic solution alone. Accordingly, if a non-fluid electrolyte is used, impregnation into the electrodes of the electrolyte further tends to be insufficient.
Further, in a cell employing the casing having a variable shape, in order to avoid e.g. swelling of the cell due to evaporation of the solvent contained in the above-mentioned electrolytic solution, it is preferred to use a solvent having a high boiling point as the solvent for the electrolytic solution. However, a solvent having a high boiling point generally has high viscosity, and accordingly, there is a problem that if such a solvent having a high boiling point is used, the fluidity of the electrolytic solution lowers, and thereby the fluidity of the electrolyte lowers, whereby the impregnation property of the electrolyte becomes poor.
If the impregnation property of an electrolyte is poor, not only it takes a long time for the impregnation, but also the initial capacity of the cell becomes insufficient, or the rate characteristics or the initial characteristics tend to be poor.
The present invention has been made in consideration of the above-mentioned problems, and it is an object of the present invention to improve the cycle characteristics of a lithium secondary cell employing a casing having a variable shape, and to improve the impregnation property of the electrolyte thereby to improve e.g. the rate characteristics.